A mod (short for "modification") is an alteration where someone, usually a player, changes some aspect (e.g. the way it looks or behaves) of a video game. Mods may range from small changes and simple tweaks to completely new games made within a video game.

Games running on a personal computer are often designed with change in mind, allowing modern PC games to be modified by gamers without much difficulty. These mods can extend the replay value and interest of the game.

The Internet provides an inexpensive medium to promote and distribute user created content like mods, an aspect commonly known as Web 2.0. Video game modding was described as remixing of games and can be therefore seen as part of the remix culture as described by Lawrence Lessig.]The Mods have become an increasingly important factor in the commercial success of some games. Developers such as id Software, Valve Corporation, Mojang AB, Bethesda Softworks, Firaxis, Crytek, The Creative Assembly and Epic Games provide extensive tools and documentation to assist mod makers, leveraging the potential success brought in by a popular mod like Counter-Strike.

Mods can help to continue the success of the original game, even when the original game has become dated. In that case, players might have to clarify that they are referring to the unmodified game when talking about playing a game. The term vanilla is often used to make this distinction. "Vanilla Battlefield 1942", for example, refers to the original, unmodified game. For vanilla games, prefix "v" or "V" is commonly used together with the game title acronym, e.g., VQ3 stands for "vanilla Quake 3".

As early as the 1980s, video game mods have been used for the sole purpose of creating art, as opposed to an actual game. They can include recording in-game actions as a film, as well as attempting to reproduce real-life areas inside a game with no regard for game play value. See also artistic video game modification, machinima, and demoscene.